Welcome aboard. As to your questions:
1. They're fairly close in that they have some similar functions but the results are way different. Gimp has one handy thing that PS doesn't...tubes (which is like a brush that has multiple tips that cycle through and each tip is its own image, sort of like a layered image). This would be handy for doing something like a bunch of different butterflies.
2. No idea on the cost but I'm guessing around $900 for the full suite or $500 for PS alone. I'm sure that the adobe site has the prices. It's on CS4...the CS versions started right after 7 and while many use CS4 there are some wonky bits in it and I still use CS from 2003.
3. As for tuts, nothing comes to mind right away for modern cities. But you can look through the tutorials section and the sci-fi/modern mapping section to see if there is anything there.